Mt Tongariro at sunset after the eruption, seen from a lookout point on Te Ponanga Saddle Rd.

There is a dark cloud hanging over Tongariro National Park, and it's not just the ash and steam from Te Maari crater eruptions.

An eruption at the crater about 1.30pm on Wednesday sent an ash plume shooting up to 4 kilometres into the sky, though the mountain quietened later in the day.

Scientists say further eruptions of Mt Tongariro could continue for months, if not years. And GNS Science last week warned of a possible eruption at Mt Ruapehu.

This morning, GNS Science duty vulcanologist Nico Fournier said there had been little change overnight, but that did not mean it wouldn't erupt again unexpectedly.

"It was a very quiet night. We haven't detected any change in activity, but having said that, it could still erupt with little or no notice," he said.

While sightseers parked up on the roadside to get a view of the eruption yesterday, tourism operators who provide accommodation, shuttle transport and guiding services were cautiously optimistic the world-renowned Tongariro Crossing would not remain closed for long.

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Only a handful of operators were willing to talk about the effect on their business from the temporary closure of the crossing, though Tongariro Alpine Transport Users Group spokesman Stew Barclay, of National Park, said trade would undoubtedly drop off.

Operators were keen to get a message out that the area offered other alternatives for tourists.

"Really the eruption at Te Maari this week was minute compared with the initial eruption back in August - probably about 10 per cent of the size," Barclay said.

"The difficulty we have is the perception [of] people, who have made or are about to make bookings, thinking that it's too risky to visit the park over the next three to four months."

His company, Adrift Guiding, had already received calls from overseas visitors either wanting to ensure it was safe to visit, or to cancel bookings.

"Some want a refund and we suggest there are many other walks and activities in National Park which are unaffected by the eruption.

"The Tongariro Crossing is famous because of what it is - a great one-day walk - but we tell people we can offer them other walks in the park, as well kayaking and mountain biking."

Barclay was optimistic the track would be partially open by Monday, and said the Conservation Department was aware of the commercial operators' plight.

Plateau Lodge and Shuttles manager Zeus Messenger said most visitors came to walk the Tongariro Crossing and it was sometimes difficult to convince them that there were other, equally good, walks in the park.

Mountain Air pilot Matthew Matthews said business had increased markedly in the past two days, with two planes fully booked with sightseers.

"We've had eight to nine flights fully booked each day."

Flying conditions were safe and pilots were keeping upwind of the steam, he said.